The use of positron emission tomography (PET) is growing in the field of medical imaging. In PET imaging, a radiopharmaceutical agent is introduced into the object to be imaged via injection, inhalation, or ingestion. After administration of the radiopharmaceutical, the physical and bio-molecular properties of the agent will cause it to concentrate at specific locations in the human body. The actual spatial distribution of the agent, the intensity of the region of accumulation of the agent, and the kinetics of the process from administration to eventually elimination are all factors that may have clinical significance. During this process, a positron emitter attached to the radiopharmaceutical agent will emit positrons according to the physical properties of the isotope, such as half-life, branching ratio, etc.
The radionuclide emits positrons, and when an emitted positron collides with an electron, an annihilation event occurs, wherein the positron and electron are destroyed. Most of the time an annihilation event produces two 511 keV gamma rays traveling at substantially 180 degrees apart.
By detecting the two gamma rays, and drawing a line between their locations, i.e., the line-of-response (LOR), one can retrieve the likely location of the original disintegration. While this process will only identify a line of possible interaction, by accumulating a large number of those lines, and through a tomographic reconstruction process, the original distribution can be estimated. In addition to the location of the two scintillation events, if accurate timing (within few hundred picoseconds) is available, a time-of-flight (ToF) calculation can add more information regarding the likely position of the event along the line. The collection of a large number of events creates the necessary information for an image of an object to be estimated through tomographic reconstruction. Two detected events occurring at substantially the same time at corresponding detector elements form a line-of-response that can be histogrammed according to their geometric attributes to define projections, or sinograms to be reconstructed. Direct reconstruction of the LORs in list-mode format without histogramming is also possible, which is usually done in the TOF case.
In PET, scintillation crystals with related detector electronics are used to detect these emitted photons, and then a 3D radioisotope distribution of the organism can be obtained. In PET, the initial data detected from scintillation crystals are single events. A pairing scheme is needed to connect two single events, whose arrival time difference is within a coincidence timing window, into coincidence events. In the coincidence events, which we call “prompts,” not all paired events are from the same annihilation points, which we call “true coincidences.” A fraction of them are due to the accidental pairing of two singles from two different annihilation points, which we call “randoms” or “accidental coincidences.”
In conventional systems, single pairing is performed using a hardware implementation of pairing in and-logic and/or with ASIC chips, while multi-coincidence is rejected for most clinical PET scanners. Further, in conventional systems, the transaxial imaging FOV is determined coarsely by detector block/module pairs.
However, conventional pairing methods have several disadvantages, including: (1) little flexibility when there is need to change pairing parameters or schemes; (2) the detector block/module pair only defines a coarse transaxial imaging FOV, and there is a need to further trim down the excess imaging FOV using other methods in later data processing; (3) there is no direct evaluation or comparison of hardware pairing with “gold standard” pairing results obtained using, e.g., GATE simulated data, and there is a need to write separate pairing code to check results with GATE simulated data; (4) a hardware implementation of pairing does not allow the same circuitry to be used for different scanner geometries and variants; thus hardware reuse between different systems is limited or non-existent, which results in high engineering development and material cost; (5) using specialized hardware (and-logic and/or ASIC chips) for pairing increases the risk of having to re-spin ASICs or boards during the development and maintenance phases if defects and/or improvements are needed, which results in a significant cost increase; and (6) the specialized hardware used for pairing cannot be used by the scanner system to perform any other functions during the periods when the pairing function is idle.